1. Technical Field
The invention relates to a fixed-focus imaging lens that can be suitably mounted on a small-sized imaging apparatus such as (i) a digital camera using an imaging device such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), (ii) a camera using a silver halide film, and the like.
2. Description of the Related Art
In recent years, as permeation of personal computers in homes or the like, a digital still camera (hereinafter, referred to as a “digital camera”) capable of inputting image information, such as photographed scenes, persons, or the like, to the personal computer have come into wide use rapidly. As cellular phones are sophisticated, module cameras (portable module cameras) for inputting images have been often mounted on the cellular phones.
In such an imaging apparatus, an imaging device such as a CCD or a CMOS is used. With the recent development in miniaturization of the imaging devices, compactness is requested for such imaging apparatus and an imaging lens mounted thereon. Concurrently, since the number of pixels of the imaging device also increases, the imaging lenses are requested to have higher resolution and higher performance.
JP 2004-302058 A (corresponding to US 2004/0190162 A), JP 2005-173319 A, JP 2005-227755 A and JP 2005-292235 A describe imaging lenses that has a three-lens configuration and that employs an aspheric lens surface, thereby achieving higher performance and compactness.
As described above, as recently miniaturization of the imaging device is further advanced and the number of pixels thereof further increases, particularly an imaging lens for a digital camera is requested to have higher resolution performance and a compact configuration. On the other hand, an imaging lens for a portable module camera has been primarily requested to be low in cost and compact in configuration. Recently, the number of pixels of an imaging device in a portable module camera also tends to increase. Thus, the imaging device in the portable module camera has been highly requested to have good performance.
It is therefore desired to develop a wide variety of lenses comprehensively improved in cost, image-formation performance and compactness. For example, it is desired to develop a low-cost and high-performance imaging lens having high performance with considering mounting it on a digital camera, while ensuring compactness for mounting on a portable module camera.
To meet these requests, for example, a lens configuration can be considered in which the number of lenses is three in order to secure compactness and low cost, and aspheric surfaces are used aggressively in order to secure high performance. In this case, the use of aspheric surfaces contributes to compactness and high performance. However, the aspheric surfaces are disadvantageous in manufacturing efficiency, and the cost may increase. It is, therefore, desired to take the manufacturing efficiency into consideration when the aspheric surfaces are used. The lenses described in the above listed documents have a three-lens configuration using aspheric surfaces. However, those lenses do not achieve a certain level of compatibility between good imaging performance and desirable compactness.